During continuous galvanization of a metal sheet, for example a steel sheet, the steel sheet continuously passes through a bath that contains molten metal, usually zinc. In the bath, the sheet usually passes below an immersed roller and then moves upwards through stabilizing and correcting rollers. The sheet leaves the bath and is conveyed through a set of gas-knives, which blow away superfluous zinc from the sheet and back to the bath to control the thickness of the coating. The gas that is blown out with the knives is usually air or nitrogen, but also steam or inert gas may be used. The sheet is then conveyed without support until the coating has been cooled down and solidified. The coated steel sheet is then led or directed via an upper roller for continued treatment of the steel sheet such as, for example, cutting of the sheet into separate sheet elements or for winding the sheet onto a roller. Normally, the sheet moves in a vertical direction away from the roller immersed into the bath through the correcting and stabilizing rollers and the gas-knives to the upper roller.
When steel sheet is galvanized, an even and thin thickness of the coating is aimed at. One common method is to measure the mass of the coating after the sheet has passed through the upper roller. This reading is utilized for controlling the gas-knives and hence controlling the thickness of the coating. The gas-knives are usually arranged suspended from a beam that is movably arranged in the vertical direction and in a direction towards the sheet. The gas-knives may also be angled such that the angle at which the gas hits the coating on the sheet may be changed. Due to the geometry of the steel sheet, the length the sheet has to run without support, its speed and the blowing effect of the gas-knives, however, the steel sheet will move or vibrate in a direction that is essentially perpendicular to its direction of transport. Certain measures, such as the use of correcting and stabilizing rollers, a precise control of the gas flow from the gas-knives, and an adjustment of the speed of the steel sheet and/or an adjustment of the distance over which the sheet has to run without support, may be taken for the purpose of reducing these transversal movements. If they are not reduced, these transversal movements will considerably disturb the exact wiping of the gas-knives, which results in an uneven thickness of the coating.
In the Japanese publication with publication number JP 09-202955, it is shown how the vibrations in a metallic sheet are reduced with the aid of rolls that stabilize and tension the sheet after having passed through the gas-knives. The position of the sheet in relation to its direction of transport in a plane is measured with a sensor, from where information is passed on to a computer that carries out a vibration analysis based on the values obtained and, together with information about the speed of the sheet, calculates the optimum tensioning of the sheet to control the vibrations in the sheet.
It is also known from, inter alia, U.S. Pat. No. 6,471,153 and JP 8010847 A to arrange, in a device for galvanizing a steel sheet, a plurality of electromagnets along the width of the sheet, which generate magnetic forces acting perpendicular to the sheet in order to damp vibrations in the sheet. A sensor measures the distance between the steel sheet and the electromagnet and a control device controls the flow of a current through the electromagnet from the distance measured by the sensor. In case of narrow widths of the sheet, the electromagnets which end up outside the edges of the sheet are shut off as the value measured by the sensors becomes incorrect since, when the electromagnets end up outside the edges of the sheet, there is no sheet between the magnets. This further means that the control Systems for this type of solution will be unnecessarily expensive and complicated. Using many magnets, as described in the above-mentioned documents, also entails increased costs, increased system complexity and a risk of introducing new unwanted oscillations.
There is a need of a cost-effective device and method for stabilizing a steel sheet, wherein the device may be used for several different widths of steel sheet without having to control certain electromagnets when the sheet width is changed.